Basically, the standard Isuzu intake manifold, even with an adapter plate, that annoying tube that connects the intake to the exhaust plugged off, all of the other smog-related fittings and connectors plugged off and/or removed, and with the EGR plugged off, has a lot worse flow and cooling than an offy, plus is prone to leaks around the adapter plates when using a weber.
The offy 6204 model is a cleaner looking intake, is custom-designed for better flow and better cooling, needs no adapter plate to bolt on a weber, does not have the EGR connections, smog-related fittings and connections, or that fitting for that annoying exhaust-to-intake tube, plus it is designed a little better where it bolts up to the head to help eliminate breakage around the bolt holes at the ends and to help prevent any warpage. Their design, engineering, casting, and machine work has always been first-class IMHO.
If anyone wants to do a search about offenhauser products here:
http://72.14.253.104/search?q=cache:YvaB4tnb6D0J:store.summitracing.com/sitemap2.asp%3FN%3D700%2B115%2B400429+offenhauser&hl=en&ct=clnk&cd=3&gl=us
or even on ebay:
http://search.ebay.com/offenhauser_W0QQ_trksidZm37QQfromZR40
they will find that an offy intake/weber carb combination has been used for racing or high-performance applications here in the USA for years on the Isuzu 1.8, 1.9, and 2.0 engines. An offy/weber combination also has been used on Datsun/Nissan, Toyota, Volkswagon, and a ton of other 4-cylinder and 6-cylinder applications. In addition, offy has made many intakes for V8 engines, and some V6's using many other carbs too for a long time. They also have a lot of other products other than intakes that they custom-make for similar applications.
As for weber, they have been in the carb business for a long time too. They have been making superior products for just about any type of engine for many years. If anyone wants to know more about them, they can look here:
http://www.redlineweber.com/
or do an ebay search:
http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=weber+carb&category0=
All things considered, you would be hard pressed to find anyone in the USA that has seriously been into high performance, racing, or off-roading for any substantial length of time that hasn't used an offy or weber product or at least competed against somebody that has.